An Arduino light sensor circuit is able to detect changes in light. This is pretty handy if you need to make a project that requires being able to tell how light or dark it currently is. For example, an automatic chicken hatch door, automatic lights etc. https://arduinomylifeup.com/arduino-light-sensor/
Program your pages. Add more control and features to your notebook by adding microcontrollers like the ATTiny85 to your paper circuitry. Make your LEDs blink in different patterns. Have them react to a light sensor. All this and more is possible with a chip, a computer, a few components, and the Arduino IDE.